The present invention relates to a process for synthetizing hydrocarbons from a mixture comprising COxe2x80x94(CO2)xe2x80x94H2 (i.e., a mixture comprising carbon monoxide and hydrogen and possibly carbon dioxide, known as synthesis gas). This process comprises using a catalyst comprising at least one group VIII metal, preferably cobalt, supported on a silica-alumina, prepared by co-precipitating and calcining at a temperature in the range from about 500xc2x0 C. to about 1200xc2x0 C. for at least 6 hours, so as to have a specific surface area of less than 260 m2/g.
The skilled person is aware that synthesis gas can be converted to hydrocarbons in the presence of a catalyst containing transition metals. Such conversion, carried out at high pressures and under pressure, is known in the literature as the Fischer-Tropsch synthesis. Metals from group VIII of the periodic table such as iron, ruthenium, cobalt and nickel catalyse the transformation of COxe2x80x94(CO2)xe2x80x94H2 mixtures (i.e., a mixture of carbon monoxide, hydrogen and possibly carbon dioxide, known as synthesis gas) to liquid and/or gaseous hydrocarbons.
Different methods have been described and developed in the prior art that are intended to improve the preparation of Fischer-Tropsch catalysts based on cobalt supported on different supports. The most widely used supports are alumina, silica and titanium dioxide, occasionally modified by additional elements.
International patent WO-A-99/39825 describes the use of a support comprising a titanium dioxide base on which a binder constituted by silica and alumina has been incorporated. The mechanical properties of the catalyst obtained are improved, in particular for use in a three-phase reactor, generally known as a slurry reactor. Usually, such a reactor is of the slurry bubble column type.
WO-A-99/42214 describes adding a stabilising element to an Al2O3 support used to prepare a catalyst that is active in the Fischer-Tropsch process.
The stabiliser can be selected from the group formed by: Si, Zr, Cu, Mn, Ba, Co, Ni and/or La. It can substantially reduce the solubility of the support in acid or neutral aqueous solutions.
Cobalt-based Fischer-Tropsch catalysts described in the invention cited above and used in a three-phase reactor can lead to excessive losses of catalyst in the paraffin waxes produced, by the formation of submicronic fines. The catalyst losses, expressed with respect to the cobalt, can reach 50 mg of cobalt per kilogram of wax.
U.S. Pat. Nos. 5 169 821 and 5 397 806 describe including silicon, zirconium or tantalum in a cobalt-based catalyst supported on TiO2 in the form of anatase to stabilise it to high temperature regeneration.
WO-A-96/19289 describes the use of a catalyst to convert synthesis gas to hydrocarbons based on cobalt, ruthenium or iron supported on a mesoporous crystalline aluminosilicate with a particular pore structure.
U.S. Pat. No. 4 497 903 describes incorporating cobalt into the crystalline layers of an aluminosilicate. The catalyst obtained is active on converting synthesis gas into liquid hydrocarbons principally constituted by branched hydrocarbons with a high octane number.
U.S. Pat. No. 5 045 519 describes a process for preparing silica-alumina leading to a high purity product that is heat stable. It is prepared by hydrolysis of an aluminium alkoxide and simultaneous or successive addition of orthosilicic acid previously purified by ion exchange. The silica-aluminas obtained are used as a support for a desulphurisation catalyst, in DeNOx catalysis, for oxidation, in hydrocracking, in mild hydrocracking, in automobile exhaust catalysis, and in isomerisation.
The present invention concerns a process for synthetizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen, and possibly carbon monoxide, in the presence of a catalyst comprising at least one group VIII metal, preferably cobalt, supported on a particular silica-alumina which will be described below. The catalyst is preferably used in suspension in a liquid phase in a perfectly stirred autoclave type three-phase reactor or slurry bubble column. It is also suitable for use in a fixed bed.
The Applicant has discovered that using a silica-alumina support prepared by co-precipitating and calcining at a high temperature for a time sufficient to encourage interactions between the alumina and the silica (Alxe2x80x94Oxe2x80x94Si bonds) can, after impregnation with at least one group VIII metal, preferably cobalt, produce a catalyst that is particularly active in a process for synthetizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen. Further, said catalyst has improved mechanical properties, in particular when, as is preferable, it is used in suspension in a liquid phase in a three-phase reactor, and it has better resistance to attrition phenomena.
The silica-alumina used in the process of the present invention is preferably a silica-alumina that is homogeneous on the micrometric scale and in which the quantity of anionic impurities (for example SO42xe2x88x92, Clxe2x88x92) and cationic impurities (for example Na+) is preferably less than 0.1% by weight, more preferably less than 0.05% by weight.
The silica-alumina used in the process of the invention is prepared by co-precipitation. By way of example, the silica-alumina support used in the process of the invention can be prepared by true co-precipitation under controlled stationary operating conditions (average pH, concentration, temperature, mean residence time) by reacting a basic silicon-containing solution, for example in the form of sodium silicate, optionally aluminium, for example in the form of sodium aluminate, with an acidic solution containing at least one aluminium salt, for example aluminium sulphate.
After co-precipitation, the support is obtained by filtering and washing, optionally washing with an ammoniacal solution to extract the residual sodium by ion exchange, drying and forming, for example by spray drying then calcining, preferably in air in a rotary oven and at a high temperature, generally in the range about 500xc2x0 C. to about 1200xc2x0 C., for a time sufficient to encourage the formation of interactions between the alumina and the silica, generally at least 6 hours. These interactions lead to a better mechanical strength of the support and thus of the catalyst used in the process of the invention.
A further method for preparing the silica-alumina of the invention consists of preparing, from a water-soluble alkaline silicate, a solution of orthosilicic acid (H4SiO4, H2O) decationised by ion exchange then mixed with a cationic aluminium salt in solution, for example the nitrate, and finally co-precipitating the solution obtained with ammonia under controlled operating conditions. After filtering and washing, drying with forming and calcining between about 500xc2x0 C. and about 1200xc2x0 C. for at least 6 hours, a silica-alumina support that can be used in the process of the invention is obtained.
A preferred method, disclosed in U.S. Pat. No. 5 045 519, consists of preparing a decationised orthosilicic acid as above then simultaneously or consecutively adding an aluminium alkoxide such as aluminium trihexanoate to the hydrolysed product. After intense homogenisation of the suspension by vigorous stirring, optional adjustment of the dry material content by filtering then re-homogenisation, the product is dried and formed, then calcined between about 500xc2x0 C. and about 1200xc2x0 C. for at least 6 hours.
In all of the preparation methods described, during any step of the preparation it may be desirable to add a small proportion of a stabilising element selected from the group formed by lanthanum, praseodymium and neodymium. The stabilising element is preferably added in the form of a soluble salt, for example a nitrate.
Preferably, a soluble salt of at least one stabilising element is added to the aqueous cationic aluminium salt or, as disclosed in U.S. Pat. No. 5 045 519, simultaneously or consecutively with addition of the orthosilicic acid to the hydrolysed product of at least one aluminium alkoxide.
The silica-alumina supports used in the present invention preferably contain between 0.5% and 30% by weight of silica, more preferably between 1% and 20% by weight, still more preferably between 1.4% and 15% by weight of silica with respect to the anhydrous product.
They can also contain 0.1% to 5% by weight, preferably 0.5% to 2% by weight, of at least one oxide M2O3 of at least one metal M selected from the group formed by lanthanum, praseodymium and neodymium.
The support is preferably formed into a fine calibrated powder with a grain size of 800 microns (xcexcm) or less, preferably in the range 10 to 500 xcexcm, more preferably in the range 10 to 300 xcexcm, still more preferably in the range 20 to 150 xcexcm, for optimum use in the presence of a liquid phase. The single drying-plus-forming step is preferably carried out by spray drying, to obtain substantially spherical microbeads with a size of less than about 800 xcexcm.
After drying, the product is calcined, for example in air and in a rotary oven at a temperature in the range about 500xc2x0 C. to about 1200xc2x0 C., preferably in the range 550xc2x0 C. to 1200xc2x0 C., more preferably in the range 700xc2x0 C. to 1200xc2x0 C., highly preferably in the range 800xc2x0 C. to 1100xc2x0 C., and for a time sufficient to bring the specific surface area to less than 260 m2/g, preferably less than 220 m2/g, more preferably to a specific surface area in the range 130 to 200 m2/g, and highly preferably in the range 130 to 190 m2/g. Said calcining step generally lasts at least 6 hours, preferably at least 10 hours, more preferably at least 15 hours. As an example, said silica-alumina can be calcined for 12 hours at 1050xc2x0 C.
It is also possible to commence calcining at low temperature, i.e., at a temperature in the range 350xc2x0 C. to 550xc2x0 C. for at least 1 hour, preferably at least 3 hours, then to raise the temperature to a temperature in the range about 500xc2x0 C. to about 1200xc2x0 C.
In a further implementation, the support is calcined firstly at 550xc2x0 C. for three hours then treated in an air/H2O mixture at 800xc2x0 C. for 24 hours, to obtain the desired specific surface area.
In a further variation, the support is in the form of spheres or extrudates with an equivalent diameter in the range 2 to 10 mm, for use in a fixed bed.
The catalyst supports for use in the process of the present invention have micronic homogeneity (i.e., on the micron scale), determined by microanalysis using a Castaing microprobe, such that the Si/Al atomic ratio, measured locally in several sections of the support particles, does not fluctuate by more that 20% around the mean value.
Preferably, the supports resulting in the catalysts used in the process of the invention have a xe2x80x9cnanometricxe2x80x9d homogeneity, i.e., on the nanometre scale.
One method that can be used to characterise the supports, and in particular to determine their homogeneity, is the transmission electron microscope (TEM). To this end, an electron microscope is used (JEOL 2010 or Philips Tecnai20F, with optional scanning) provided with an energy dispersion spectrometer (EDS) for X ray analysis (for example a Tracor or an Edax). The EDS detector must allow light element detection. The combination of these tools, TEM and EDS, combines imagery and local chemical analysis with good spatial resolution.
For this type of analysis, the samples are finely and ground in a mortar; the powder is then incorporated into a resin to produce ultrafine sections about 70 nanometres (nm) thick. These sections are collected on copper screens coated with an amorphous perforated carbon film, and acting as a support. They are then introduced into the microscope for observation and analysis under high vacuum. The image allows the sample zones to be readily distinguished from the resin zones. A certain number of analyses, a minimum of 10, preferably in the range 15 to 30, are then carried out on different zones of the incorporated sample. The diameter of the electron beam for analysing the zones (approximately determining the size of the analysed zones) is a maximum of 50 mm, preferably 20 nm, more preferably 10, 5, 2 or 1 nm in diameter. In scanning mode, the analysed zone will be a function of the size of the scanned zone and not the size of the beam, which is generally reduced.
Semi-quantitative treatment of the X ray spectra recovered using the EDS spectrometer can provide the relative concentration of Al and Si (in atomic %) and the ratio Si/Al for each of the zones analysed. The mean ratio, Si/Alm, can be calculated, along with the standard deviation "sgr" of this set of measurements.
The process of the present invention uses catalysts wherein the relative standard deviation "sgr" (with respect to the value Si/Alm) is less than 30%, preferably 20%, for an overall Si/Al that is preferably in the range 0.1 to 10. This overall ratio can be measured by other techniques that are routinely used for this type of analysis (X ray fluorescence, for example).
The catalyst used in the process of the invention is constituted by at least one element from group VIII (element selected from elements from groups 8, 9 and 10 in the new periodic table) supported on a silica-alumina with the characteristics described above, and prepared by co-precipitation.
The element from group VIII of the periodic table is preferably selected from iron, cobalt and ruthenium. More preferably, the group VIII metal is cobalt.
One technique for preparing the catalyst which is particularly suitable for carrying out the process of the invention is impregnation of an aqueous solution of a precursor of the metal from group VIII of the periodic table, preferably cobalt, for example an aqueous solution of salts such as cobalt nitrates. The weight content of the group VIII metal with respect to the total catalyst weight is generally in the range 0.1% to 50%, preferably in the range 1% to 30%.
The catalyst an also contain at least one additional element, for example an activity promoter, for example at least one element selected from the group formed by ruthenium, molybdenum and tantalum, or reducibility promoters such as platinum, palladium or ruthenium. The amount by weight of additional element with respect to the total catalyst weight is generally in the range 0.01% to 5%. These additional elements are preferably introduced at the same time as the group VIII metal or, in a preferred variation, in at least one subsequent step.
In a particular implementation of the invention, the catalyst contains both cobalt and ruthenium. In a further particular implementation of the invention, the catalyst contains cobalt and tantalum.
The mechanical strength of the catalyst of the invention is improved with respect to a catalyst comprising a support constituted solely by alumina or silica, or titanium dioxide.
The mechanical strength of the catalyst of the invention can be determined by measuring the change in the particle size at the end of a set test period during use of a three-phase reactor.
The catalysts so prepared result in particularly stable performances in Fischer-Tropsch synthesis and to a conversion of synthesis gas into a mixture of linear and saturated hydrocarbons containing at least 50% by weight of C5+ hydrocarbons and less than 20% of methane with respect to the total hydrocarbons formed.
The following conditions are normally employed for said catalysts when synthetizing hydrocarbons:
The catalyst comprising at least one group VIII metal impregnated on the silica-alumina support described above is dried then calcined. The catalyst is then pre-reduced using at least one reducing compound, for example selected from the group formed by hydrogen, carbon monoxide and formic acid, optionally mixed with an inert gas, for example nitrogen, in a reducing compound/(reducing compound+inert gas) mole ratio in the range 0.001:1 to 1:1.
Reduction can be carried out in the gas phase at a temperature in the range 100xc2x0 C. to 600xc2x0 C., preferably in the range 150xc2x0 C. to 400xc2x0 C., at a pressure in the range 0.1 to 10 MPa and at an hourly space velocity in the range 100 to 40000 volumes of mixture per volume of catalyst per hour.
This reduction can also be carried out in the liquid phase, under the same operating conditions as in the gas phase, the catalyst then being suspended in an inert liquid phase (also known as the solvent), for example a paraffin cut comprising at least one hydrocarbon containing at least 5, preferably at least 10 carbon atoms per molecule.
When the catalyst is used in a three-phase reactor, it may be advantageous to use, as is preferable, the same inert solvent as that used during the reaction. Highly preferably, a paraffin cut from the Fischer-Tropsch process is used, for example a kerosene or diesel cut. Preferably, this reduction is carried out in situ, i.e., in the reactor that is then used to carry out the Fischer-Tropsch synthesis.
The catalyst used in the process of the invention can also be reduced ex situ or offsite, i.e., not in the Fischer-Tropsch synthesis reactor, or even outside the industrial site carrying out the process. Reduction can then optionally be carried out by an enterprise accustomed to carrying out offsite treatments.
In such a case, the catalyst is reduced under the operating conditions described above. After reducing and cooling the reduced catalyst to at least 100xc2x0 C., said catalyst is preferably mixed, in an amount of 10% to 80% by weight, with solid paraffin waxes at ambient temperature and pre-heated to liquefy the waxes. Preferably, paraffin waxes from a Fischer-Tropsch process are used. After mixing, the suspension obtained is drop coagulated by projecting onto a support belt followed by cooling. The product obtained is in the form of grains with an equivalent diameter (diameter of the sphere with a equivalent volume) in the range about 5 to about 20 mm in diameter. These catalyst grains can be charged directly into the Fischer-Tropsch reactor.
The conversion of synthesis gas to hydrocarbons is then carried out under a total pressure that is normally in the range 0.1 to 15 MPa, preferably in the range 1 to 10 MPa; the temperature is generally in the range 150xc2x0 C. to 350xc2x0 C., preferably in the range 170xc2x0 C. to 300xc2x0 C. The hourly space velocity is normally in the range 100 to 20000 volumes of synthesis gas per volume of catalyst per hour (hxe2x88x921), preferably in the range 200 to 10000 hxe2x88x921, more preferably in the range 400 to 5000 hxe2x88x921, and the H2/CO ratio in the synthesis gas is normally in the range 1:2 to 5:1, preferably in the range 1.2:1 to 2.5:1.
The catalyst may be used in the form of a fine calibrated powder with a grain size of less than 800 microns (xcexcm), preferably in the range 10 to 500 xcexcm, more preferably in the range 10 to 300 xcexcm, and highly preferably in the range 20 to 150 xcexcm, most preferably in the range 20 to 120 xcexcm, when it is used in suspension in a liquid phase. It can also be used in the form of particles with an equivalent diameter in the range about 2 to 10 mm, preferably in the range 3 to 8 mm, when used in a fixed bed.
The process of the present invention can be used with said catalyst disposed in a fixed bed. In such a process, the reaction takes place in the gas phase. The mechanical strength of the catalyst described above is sufficiently high for it to be manipulated and charged into such a reactor without risk of disintegration. The process of the invention can also, as is preferable, be carried out in a three-phase reactor in which the catalyst is in suspension in an inert liquid phase (solvent). As an example, a perfectly stirred reactor such as an autoclave or a bubble column type three-phase reactor (also known as a slurry bubble column) can be used.
The catalyst is advantageously used in a three-phase reactor, preferably a slurry bubble column, as this type of operation enables:
optimum use of the performances of the catalyst (activity and selectivity), by limiting intra-granular diffusion phenomena;
substantial limitation of thermal effects in the catalyst grain, which is surrounded by a liquid phase.
This type of implementation requires that the catalyst and reaction products be separated.
Under these conditions, the catalyst used in the process of the invention has improved mechanical properties, which leads to optimum separation of the catalyst and products, and an improved service life. Said catalyst has an improved attrition resistance, and thus a substantial reduction in the amount of fines formed during use with a three-phase reactor. One possible explanation for this improvement is the presence of more substantial and a greater number of interactions between the alumina and the silica in the silica-alumina prepared by co-precipitation.
In summary, the invention concerns a process for synthetizing hydrocarbons from a mixture comprising carbon monoxide and hydrogen in the presence of a catalyst comprising at least one group VIII metal supported on a silica-alumina prepared by co-precipitating and calcining at a temperature in the range from about 500xc2x0 C. to about 1200xc2x0 C. for at least 6 hours such that said silica-alumina has a specific surface area of less than 260 m2/g.
In a preferred implementation, the silica-alumina is calcined at a temperature in the range 700xc2x0 C. to 1200xc2x0 C. for at least 10 hours. In a further preferred implementation, the silica-alumina is initially calcined at a temperature in the range about 350xc2x0 C. to about 550xc2x0 C. for at least 1 hour, then at a temperature in the range about 500xc2x0 C. to about 1200xc2x0 C. for at least 6 hours.
Preferably, the silica-alumina is homogeneous on the micrometre scale, and more preferably, the amount of anionic and cationic impurities is less than 0.1% by weight.
Preferably, the silica-alumina contains 0.5% to 30% by weight of silica with respect to the anhydrous product, and the group VIII metal content is in the range 0.1% to 50% by weight. The group VIII metal is preferably cobalt.
The catalyst for the process of the invention can optionally also contain at least one additional element selected from the group formed by: ruthenium, molybdenum, tantalum, platinum and palladium. It can also contain 0.1% to 5% by weight of at least one oxide M2O3 of at least one metal M selected from the group formed by lanthanum, praseodymium and neodymium.
Preferably, the catalyst is used in suspension in a liquid phase, in a three-phase reactor, generally in the form of a powder with a grain size of less than 800 microns. Said catalyst can, however, be used in a fixed bed in the form of particles with an equivalent diameter in the range 2 to 10 mm.
The following examples illustrate the invention.